Methods and apparatus for reducing the effects of DAC images in radio frequency transceivers

Abstract

Methods and apparatus for reducing the effects of digital-to-analog converter (DAC) images and transmission spurious effects in a receive frequency band of a radio frequency (RF) transceiver. A transceiver apparatus includes a transmitter portion having a DAC, a receiver portion configured to receive RF signals in a receive frequency band, and a variable rate clock generator. The variable rate clock generator is used to provide an oversampling clock for the DAC. The rate of the oversampling clock is adjustable and is selected so that an upconverted version of a DAC image created by the DAC is steered away from frequencies within the receive frequency band. A notch-effect low-pass filter (NELPF) may also, or alternatively, be used in the transceiver to reduce transmission spurious effects in the receive frequency band.

1 . A transceiver, comprising:
 a transmitter portion including a digital-to-analog converter (DAC);   a receiver portion configured to receive RF signals in a receive frequency band; and   a variable rate clock generator operable to control said DAC,   wherein the rate of a variable rate clock provided by the variable rate clock generator is adjustable so that an upconverted version of a DAC image created by said DAC is shifted in frequency away from frequencies within said receive frequency band.   
     
     
         2 . The transceiver of  claim 1 , further comprising a data rate converter configured to receive digital data and provide rate converted digital data to said DAC. 
     
     
         3 . The transceiver of  claim 2  wherein the data rate converter provides said rate converted data to said DAC according to the variable rate clock provided by the variable rate clock generator. 
     
     
         4 . The transceiver of  claim 1  wherein the transmitter portion further includes a digital filter that functions as a notch filter, said digital filter configured so that its notch frequency falls within the receive frequency band. 
     
     
         5 . The transceiver of  claim 4  wherein the digital filter is controlled by the variable rate clock provided by the variable rate clock generator. 
     
     
         6 . The transceiver of  claim 4 , further comprising a data rate converter configured to receive digital data and provide rate converted digital data to said DAC. 
     
     
         7 . The transceiver of  claim 6  wherein the data rate converter provides said rate converted data according to the variable rate clock provided by the variable rate clock generator. 
     
     
         8 . The transceiver of  claim 6  wherein the functionality of the data rate converter is merged with the functionality of the digital filter. 
     
     
         9 . A method of processing signals in a transceiver, comprising:
 receiving a digital baseband signal from a digital source;   converting the digital baseband signal to an analog baseband signal according to an adjustable oversampling clock; and   upconverting the analog baseband signal to a radio frequency (RF) signal,   wherein converting the digital baseband signal to an analog baseband signal includes adjusting the rate of the adjustable oversampling clock so that an image created by the conversion process does not fall within a receive frequency band when the image is upconverted to RF by the upconversion process.   
     
     
         10 . The method of  claim 9 , further comprising filtering the digital baseband signal using a digital low-pass filter that functions as a notch filter. 
     
     
         11 . The method of  claim 10  wherein a notch frequency of said the digital low-pass filter is configured so that it falls within the receive frequency band. 
     
     
         12 . The method of  claim 11  wherein the notch frequency of the digital low-pass filter is controlled by the adjustable oversampling clock. 
     
     
         13 . A transceiver capable of transmitting and receiving in different transmit and frequency bands, comprising:
 a polar modulation transmitter having a magnitude path and a phase path, at least one of said magnitude and phase paths having a digital-to-analog converter (DAC) configured to receive digital baseband signals from a digital source;   a receiver that can be configured to receive radio frequency (RF) signals in any one of several possible receive frequency bands; and   an oversampling clock generator operable to provide an oversampling clock to a clock input of said DAC, wherein the oversampling clock provided by the oversampling clock generator is adjustable so that a DAC image created by the DAC and upconverted to RF is shifted away from the receive frequency band in which the receiver is configured to receive.   
     
     
         14 . The transceiver of  claim 13  wherein at least one of said magnitude and phase paths of said transmitter includes a digital filter that operates as a notch filter. 
     
     
         15 . The transceiver of  claim 14  wherein the digital filter is configured so that its notch frequency falls within the frequency band in which the receiver is configured to receive. 
     
     
         16 . The transceiver of  claim 14  wherein the digital filter is controlled by the variable rate oversampling clock provided by the oversampling clock generator. 
     
     
         17 . A mobile communication device, comprising:
 baseband circuitry operable to generate digital symbols;   a transmitter having a digital-to-analog converter (DAC) that is operable to convert the digital symbols to analog signals;   a receiver configured to receive RF signals in one or more receive frequency bands; and   means for frequency shifting an upconverted version of a DAC image created by the DAC to a frequency region that does not substantially overlap with a receive frequency band in which the receiver is configured to receive.   
     
     
         18 . The mobile communication device of  claim 17  wherein said means for shifting a DAC image is configured to frequency shift the DAC image by an amount that depends on which one of said one or more receive frequency bands the receiver is configured to receive. 
     
     
         19 . The mobile communication device of  claim 18  wherein the transmitter further comprises a digital filter configured to filter the digital symbols, said digital filter having an adjustable notch frequency. 
     
     
         20 . The mobile communication device of  claim 19  wherein the digital filter is controlled by a sampling clock provided by the means for frequency shifting a DAC image. 
     
     
         21 . A transceiver, comprising:
 a transmitter portion including a digital-to-analog converter (DAC);   a receiver portion configured to receive RF signals in a receive frequency band; and   a variable rate clock generator operable to control said DAC,   wherein the rate of a variable rate clock provided by the variable rate clock generator is determined based on an operating band of a standard the transceiver is currently configured to operate in.